<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .red {
            width: 100px;
            height: 100px;
            background-color: red;
        }
        .green {
            width: 100px;
            height: 100px;
            background-color: green;
        }
        .red-name {
            color: red;
        }
        .font-size {
            font-size: 20px;
        }
        .box {
            width: 200px;
            height: 200px;
        }
        .box-red {
            background-color: red;
        }
        .box-blue {
            background-color: blue;
        }
    </style>
</head>
<body>
    <div class="red">红色</div>
    <div class="green">绿色</div>
    <div class="red">红色</div>

    
    <div class="red-name font-size">刘德华</div>
    <!-- 多类名选择，使用空格间隔 -->
    <div class="box box-red">红色</div>
    <div class="box box-blue">绿色</div>
    <div class="box box-red">红色</div>
</body>
</html>